Abstract:

A coding apparatus capable of coding a spectrum at a low bit rate and with
high quality without producing any disturbance in a harmonic structure of
the spectrum. In this apparatus, internal state setting section sets an
internal state of a filtering section using a first spectrum S1(k). A
pitch coefficient setting section outputs a pitch coefficient T by
gradually changing it. The filtering section calculates an estimated
value S'2(k) of a second spectrum S2(k) based on a pitch coefficient T. A
search section calculates the degree of similarity between S2(k) and
S'2(k). At this time, pitch coefficient T' corresponding to the maximum
calculated degree of similarity is given to a filter coefficient
calculation section. The filter coefficient calculation section
determines a filter coefficient βi using this pitch coefficient
T'.

Claims:

1. A spectrum coding apparatus that encodes a spectrum including a first
band and a second band, the spectrum coding apparatus comprising:a
generation section that calculates an estimated spectrum of the second
band using a spectrum of the first band having at least a bandwidth of
the second band;a search section that searches for a parameter indicating
the estimated spectrum having a highest similarity to a spectrum of the
second band; anda coding section that encodes the parameter indicating
the estimated spectrum having the highest similarity, instead of the
spectrum of the second band.

2. The spectrum coding apparatus according to claim 1, wherein:the
parameter indicates a position of the spectrum of the first band apart
from the spectrum of the second band by a value within a predetermined
range; andthe generation section generates the estimated spectrum by
sequentially copying the spectrum of the first band the value apart.

3. The spectrum coding apparatus according to claim 1, wherein the search
section determines the parameter indicating the estimated spectrum having
the highest similarity by changing the parameter little by little within
a predetermined range.

4. The spectrum coding apparatus according to claim 1, wherein the search
section determines the parameter that minimizes distortion between the
spectrum of the second band and the estimated spectrum.

5. The spectrum coding apparatus according to claim 1, wherein:the
similarity is represented by a ratio between an energy of the estimated
spectrum, and a square of a cross-correlation value between the spectrum
of the second band and the estimated spectrum; andthe search section
determines a parameter that maximizes the ratio.

6. The spectrum coding apparatus according to claim 1, wherein:the first
band is a low frequency band lower than a predetermined threshold; andthe
second band is a high frequency band equal to or higher than the
predetermined threshold.

7. The spectrum coding apparatus according to claim 6, wherein the coding
section encodes envelope information of a spectrum of the high frequency
band.

8. The spectrum coding apparatus according to claim 6, wherein the coding
section encodes information relating to a power ratio between a spectrum
of the low frequency band and a spectrum of the high frequency band.

10. A base station apparatus comprising the spectrum coding apparatus
according to claim 1.

11. A scalable coding apparatus that encodes a voice signal or audio
signal separated into a low frequency band and high frequency band, the
scalable coding apparatus comprising:a first coding section that encodes
a low frequency band signal of the voice signal or the audio signal;a
second coding section that encodes a high frequency band signal of the
voice signal or the audio signal;a first spectrum generation section that
performs frequency domain conversion of the low frequency band signal and
generates a first spectrum of the low frequency band; anda second
spectrum generation section that performs frequency domain conversion of
the voice signal or the audio signal, and generates a second spectrum
including the low frequency band and the high frequency band,wherein the
second coding section comprises:a generation section that calculates an
estimated spectrum of the high frequency band of the second spectrum
using the first spectrum;a search section that searches for a parameter
indicating the estimated spectrum having a highest similarity to the high
frequency band of the second spectrum; anda coding section that encodes
the parameter indicating the estimated spectrum having the highest
similarity, instead of the high frequency band of the second spectrum.

12. A spectrum decoding apparatus comprising:a spectrum acquisition
section that acquires a spectrum of a low frequency band out of a
spectrum including the low frequency band and a high frequency band;a
parameter acquisition section that acquires a parameter indicating an
estimated spectrum that is generated using the spectrum of the low
frequency band and that has a highest similarity to a spectrum of the
high frequency band associated with an original signal; anda decoding
section that decodes the spectrum of the low frequency band and the
spectrum of the high frequency band using the spectrum of the low
frequency band and the parameter.

13. The spectrum decoding apparatus according to claim 12, wherein:the
parameter indicates a position of the spectrum of the low frequency band
apart from the spectrum of the second band by a value within a
predetermined range; andthe decoding section generates the spectrum of
the high frequency band by sequentially copying the spectrum of the low
frequency band the value apart.

14. The spectrum decoding apparatus according to claim 12, further
comprising an envelope information acquisition section that acquires
envelope information of the spectrum of the high frequency band,wherein
the decoding section performs the decoding using the envelope
information.

16. A base station apparatus comprising the spectrum decoding apparatus
according to claim 12.

17. A spectrum coding method for encoding a spectrum including a first
band and a second band, the coding method comprising:a generating step of
calculating an estimated spectrum of the second band using a spectrum of
the first band having at least a bandwidth of the second band;a searching
step of searching for a parameter indicating the estimated spectrum
having a highest similarity to a spectrum of the second band; anda coding
step of encoding the parameter indicating the estimated spectrum having
the highest similarity, instead of the spectrum of the second band.

18. A spectrum decoding method comprising:a spectrum acquiring step of
acquiring a spectrum of a low frequency band out of a spectrum including
the low frequency band and a high frequency band;a parameter acquiring
step of acquiring a parameter indicating an estimated spectrum that is
generated using the spectrum of the low frequency band and that has a
highest similarity to a spectrum of the high frequency band associated
with an original signal; anda decoding step of decoding the spectrum of
the low frequency band and the spectrum of the high frequency band using
the spectrum of the low frequency band and the parameter.

Description:

[0001]This is a continuation application of application Ser. No.
10/571,761 filed Mar. 14, 2006, which is a national stage of
PCT/JP2004/013455 filed Sep. 15, 2004, which is based on Japanese
Application No. 2003-323658 filed Sep. 16, 2003, the entire contents of
each of which are incorporated by reference herein.

TECHNICAL FIELD

[0002]The present invention relates to a coding apparatus mounted on a
radio communication apparatus or the like for coding a voice signal,
audio signal or the like and a decoding apparatus for decoding this coded
signal.

BACKGROUND ART

[0003]A coding technology for compressing a voice signal, audio signal or
the like to a low bit rate signal is particularly important from the
standpoint of effectively using a transmission path capacity (channel
capacity) of radio waves or the like and a recording medium in a mobile
communication system.

[0004]Examples of a voice coding scheme for coding a voice signal include
schemes like G726, G729 standardized by the ITU-T (International
Telecommunication Union Telecommunication Standardization Sector). These
schemes use narrow band signals (300 Hz to 3.4 kHz) as coding targets and
can perform high quality coding at bit rates of 8 kbits/s to 32 kbits/s.
However, since such a narrow band signal is so narrow that its frequency
band is a maximum of 3.4 kHz, the quality thereof is such that it gives
the user an impression that a sound is muffled, which results in a
problem that it lacks a sense of realism.

[0005]Furthermore, there is also a voice coding scheme that uses wideband
signals (50 Hz to 7 kHz) as coding targets. Typical examples of this are
6722, 6722.1 of ITU-T and AMR-WB of 3GPP (The 3rd Generation Partnership
Project). These schemes can perform coding of wideband voice signals at a
bit rate of 6.6 kbits/s to 64 kbits/s. However, when the signal to be
coded is voice, although a wideband signal has relatively high quality,
it is not sufficient when an audio signal is the target or a voice signal
of higher quality with a sense of realism is required.

[0006]On the other hand, when a maximum frequency of a signal is generally
on the order of 10 to 15 kHz, it is possible to obtain a sense of realism
equivalent to FM radio, and when the maximum frequency is on the order of
up to 20 kHz, it is possible to obtain quality comparable to that of CD
(compact disk). For such a signal, audio coding represented by the layer
III scheme or AAC scheme standardized by MPEG (Moving Picture Expert
Group) is appropriate. However, these audio coding schemes have a wide
frequency band of a signal to be coded, which results in a problem that
the bit rate of a coded signal increases.

[0007]Examples of conventional coding technologies include a technology of
coding a signal with a wide frequency band at a low bit rate (e.g., see
Patent Document 1). According to this, an input signal is divided into a
signal of a low-frequency domain and a signal of a high-frequency domain,
the spectrum of the signal of the high-frequency domain is replaced by
the spectrum of the signal of the low-frequency domain and coded, and the
overall bit rate is thereby reduced.

[0008]FIG. 1A to FIG. 1D show an overview of the above described
processing of replacing the spectrum of high-frequency domain by the
spectrum of the low-frequency domain. This processing is originally
intended to be performed in combination with coding processing, but for
simplicity of explanation, a case where the above described processing is
performed on an original signal will be explained as an example.

[0009]FIG. 1A shows a spectrum of an original signal whose frequency band
is restricted to 0≦k<FH, FIG. 1B shows a spectrum of the signal
restricted to 0≦k<FL (where, FL<FH), FIG. 1C shows a
spectrum obtained by replacing a high-frequency domain (high-frequency
band) by a low-frequency domain (low-frequency band) using the above
described technology and FIG. 1D shows a spectrum obtained by shaping the
replacing spectrum according to spectrum envelope information about the
replaced spectrum. In these figures, the horizontal axis shows a
frequency and the vertical axis shows intensity of a spectrum.

[0010]In this technology, a spectrum of the original signal whose
frequency band is 0≦k<FH (FIG. 1A) is expressed using a
low-frequency spectrum whose frequency band is 0≦k<FL (FIG.
1B). More specifically, the high-frequency spectrum (FL≦k<FH)
is replaced by the low-frequency spectrum (0≦k<FL). As a result
of this processing, the spectrum as shown in FIG. 1C is obtained. Here,
for simplicity of explanations, a case with a relationship of FL=FH/2
will be explained as an example. According to information about a
spectrum envelope of the original signal, the amplitude value of the
spectrum in the high-frequency domain of the spectrum in FIG. 1C is
adjusted and the spectrum as shown in FIG. 1D is obtained. This is the
spectrum which is the spectrum obtained by estimating the original
signal. [0011]Patent Document 1: National Publication of International
Patent Application No. 2001-521648 (pp. 15, FIG. 1, FIG. 2)

DISCLOSURE OF INVENTION

Problems to be Solved by the Invention

[0012]Generally, spectra such as voice signal and audio signal are known
to have a harmonic structure in which a peak of spectrum appears at every
integer multiple of a certain frequency (every predetermined pitch). This
harmonic structure is important information to keep the quality of a
voice signal, audio signal or the like, and if disturbance occurs in the
harmonic structure, a listener perceives deterioration of the quality.

[0013]FIG. 2A and FIG. 2B are diagrams illustrating problems of the
conventional technology.

[0014]FIG. 2A is a spectrum obtained by analyzing the spectrum of an audio
signal. As is appreciated from this figure, the original signal has a
harmonic structure having an interval T on the frequency axis. On the
other hand, FIG. 2B shows a spectrum obtained as a result of estimating
the spectrum of the original signal according to the above described
technology. When these two spectra are compared, it is observed from the
spectrum shown in FIG. 2B that the harmonic structure is maintained in
low-frequency spectrum S1 of the replacement source and high-frequency
spectrum S2 of the replacement destination, whereas the harmonic
structure is collapsed in the connection domain (spectrum S3) between
low-frequency spectrum S1 and high-frequency spectrum S2.

[0015]When this estimated spectrum is converted to a time signal and
listened, there is a problem that the listener perceives deterioration in
quality due to such disturbance of the harmonic structure, This
disturbance of the harmonic structure is caused by the fact that
replacement has been performed with no consideration given to the shape
of the harmonic structure.

[0016]It is an object of the present invention to provide a coding
apparatus capable of coding a spectrum at a low bit rate and with high
quality without producing disturbance in the harmonic structure of the
spectrum and a decoding apparatus capable of decoding this coded signal.

Solutions to the Problem

[0017]The coding apparatus of the present invention adopts a configuration
comprising an acquisition section that acquires a spectrum divided into
two bands of low-frequency band and high-frequency band, a calculation
section that calculates a parameter indicating the degree of similarity
between the acquired spectrum of the low-frequency band and the acquired
spectrum of the high-frequency band based on the harmonic structure of
the spectrum and a coding section that encodes the calculated parameter
indicating the degree of similarity instead of the acquired spectrum of
the high-frequency band,

[0018]The decoding apparatus of the present invention adopts a
configuration comprising a spectrum acquisition section that acquires the
spectrum of the low-frequency band out of the spectrum divided into two
bands of low-frequency band and high-frequency band, a parameter
acquisition section that acquires a parameter indicating the degree of
similarity between the spectrum of the low-frequency band and the
spectrum of the high-frequency band and a decoding section that decodes
the spectra of the low-frequency band and high-frequency band using the
acquired spectrum of the low-frequency band and the parameter.

[0019]The coding method of the present invention comprises an acquiring
step of acquiring a spectrum divided into two bands of low-frequency band
and high-frequency band, a calculating step of calculating a parameter
indicating the degree of similarity between the acquired spectrum of the
low-frequency band and the acquired spectrum of the high-frequency band
based on a harmonic structure of the spectrum and a coding step of coding
the calculated parameter indicating the degree of similarity instead of
the acquired spectrum of the high-frequency band.

[0020]The decoding method of the present invention comprises a spectrum
acquiring step of acquiring a spectrum of a low-frequency band out of a
spectrum divided into two bands of the low-frequency band and
high-frequency band, a parameter acquiring step of acquiring a parameter
indicating the degree of similarity between the spectrum of the
low-frequency band and the spectrum of the high-frequency band and a
decoding step of decoding the spectra of the low-frequency band and
high-frequency band using the acquired spectrum of the low-frequency band
and the parameter.

Advantageous Effect of the Invention

[0021]The present invention is capable of performing coding of a spectrum
at a low bit rate and with high quality without any collapse of a
harmonic structure of the spectrum.. Furthermore, the present invention
is also capable of improving sound quality when decoding this coded
signal.

BRIEF DESCRIPTION OF DRAWINGS

[0022]FIG. 1 is a diagram illustrating an overview of a conventional
processing of replacing a spectrum of high-frequency domain by a spectrum
of a low-frequency domain;

[0023]FIG. 2 is a diagram illustrating a problem of the conventional
technology;

[0024]FIG. 3 is a block diagram showing the principal configuration of a
radio transmission apparatus according to Embodiment 1;

[0025]FIG. 4 is a block diagram showing the internal configuration of a
coding apparatus according to Embodiment 1;

[0026]FIG. 5 is a block diagram showing the internal configuration of a
spectrum coding section according to Embodiment 1;

[0027]FIG. 6 is a diagram illustrating an overview of filtering processing
of a filtering section according to Embodiment 1;

[0028]FIG. 7 is a diagram illustrating how a spectrum of an estimated
value of a second spectrum changes as pitch coefficient T changes;

[0029]FIG. 8 is a diagram illustrating how a spectrum of an estimated
value of a second spectrum changes as pitch coefficient T changes;

[0030]FIG. 9 is a flow chart showing an example of a series of algorithms
of processes carried out by the filtering section, search section and
pitch coefficient setting section according to Embodiment 1;

[0031]FIG. 10 is a block diagram showing the principal configuration of a
radio reception apparatus according to Embodiment 1;

[0032]FIG. 11 is a block diagram showing the internal configuration of a
decoding apparatus according to Embodiment 1;

[0033]FIG. 12 is a block diagram showing the internal configuration of a
spectrum decoding section according to Embodiment 1;

[0034]FIG. 13 is a diagram illustrating a decoded spectrum generated by a
filtering section according to Embodiment 1;

[0035]FIG. 14A is a block diagram showing the principal configuration of
the transmitting side when the coding apparatus according to Embodiment 1
is applied to a wired communication system;

[0036]FIG. 14B is a block diagram showing the principal configuration of
the receiving side when the decoding apparatus according to Embodiment 1
is applied to a wired communication system.

[0037]FIG. 15 is a block diagram showing the principal configuration of a
spectrum coding section according to Embodiment 2;

[0038]FIG. 16 is a diagram illustrating an overview of filtering using a
filter according to Embodiment 2;

[0039]FIG. 17 is a block diagram showing the principal configuration of a
spectrum coding section according to Embodiment 3;

[0040]FIG. 18 is a block diagram showing the principal configuration of a
spectrum decoding section according to Embodiment 4; and

[0041]FIG. 19 is a block diagram showing the principal configuration of a
spectrum decoding section according to Embodiment 5.

BEST MODE FOR CARRYING OUT THE INVENTION

[0042]The inventor focused attention on the characteristics such as voice
signal, audio signal or the suchlike (hereinafter, collectively referred
to as "acoustic signal"), that is to say, on the fact that an acoustic
signal forms a harmonic structure in the frequency axis direction,
discovered the possibility of performing coding spectra of the remaining
bands using spectra of some bands out of spectra of all frequency bands,
and came up with the present invention.

[0043]That is, the essence of the present invention is to determine, for
example, when coding a signal spectrum divided into two frequency bands
of high-frequency domain and low-frequency domain, the degree of
similarity between the spectra of both the high-frequency domain and
low-frequency domain for the spectrum of the high-frequency domain and
perform coding of a parameter indicating this degree of similarity.

[0044]With reference to the accompanying drawings, embodiments of the
present invention will be explained in detail below.

Embodiment 1

[0045]FIG. 3 is a block diagram showing the principal configuration of
radio transmission apparatus 130 when a radio coding apparatus according
to Embodiment 1 of the present invention is mounted on the transmitting
side of a radio communication system.

[0048]F1G. 4 is a block diagram showing the internal configuration of
above described coding apparatus 120. Here, a ease where hierarchical
coding (scalable coding) is performed will be explained as an example.

[0050]A signal having an effective frequency band of 0≦k<FH is
input from A/D conversion apparatus 132 to input terminal 121.
Downsampling section 122 applies downsampling to the signal input via
input terminal 121, generates a signal having a low sampling rate and
outputs the signal. First layer coding section 123 encodes this
downsampled signal, outputs the obtained code to multiplexing section
(multiplexer) 127 and also outputs the obtained code to first layer
decoding section 124. First layer decoding section 124 generates a
decoded signal of a first layer based on the code. Upsampling section 125
increases the sampling rate of the decoded signal of first layer coding
section 123,

[0051]On the other hand, delay section 126 provides a delay of a
predetermined length to the signal input via input terminal 121. Suppose
the length of this delay has the same value as a time delay produced when
the signal is passed through downsampling section 122, first layer coding
section 123, first layer decoding section 124 and upsampling section 125.
Spectrum coding section 100 performs spectrum coding using the signal
output from upsampling section 125 as a first signal and the signal
output from delay section 126 as a second signal and outputs the
generated code to multiplexing section 127. Multiplexing section 127
multiplexes the code obtained from first layer coding section 123 with
the code obtained from spectrum coding section 100 and outputs the
multiplexed parameter as an output code via output terminal 128. This
output code is given to RF modulation apparatus 133.

[0054]The first signal is input from upsampling section 125 to input
terminal 102. This first signal is a signal which is decoded by first
layer decoding section 124 using a coded parameter coded by first layer
coding section 123 and has an effective frequency band of
0≦k<FL. Furthermore, the second signal having an effective
frequency band of 0≦k<FH (FL<FH) is input from delay section
126 to input terminal 103.

[0056]Internal state setting section 106 sets the internal state of a
filter used in filtering section 107 using first spectrum S1(k) having an
effective frequency band of 0≦k<FL. This setting will be
explained later again.

[0057]Pitch coefficient setting section 109 outputs pitch coefficients T
to filtering section 107 one by one while changing them little by little
within a predetermined search range of Tmin to Tmax.

[0058]Filtering section 107 performs filtering of the second spectrum
based on the internal state of the filter set by internal state setting
section 106 and pitch coefficient T output from pitch coefficient setting
section 109 and calculates estimated value S'2(k) of the first spectrum.
Details of this filtering processing which will be described later.

[0059]Search section 108 calculates a degree of similarity which is a
parameter indicating similarity between second spectrum S2(k) output from
frequency domain conversion section 105 and estimated value S'2(k) of the
second spectrum output from filtering section 107. This degree of
similarity will be described in detail later. Calculation processing of
this degree of similarity is performed every time pitch coefficient T is
given from pitch coefficient setting section 109 and pitch coefficient
T'(range of Tmin to Tmax) whereby the calculated degree of
similarity becomes a maximum is given to filter coefficient calculation
section 110.

[0063]Here, suppose spectra of all frequency bands (0≦k<FH) are
called "S(k)" for convenience and a filter function expressed by the
following equation will be used.

P ( z ) = 1 1 - i = - M M β i z - T +
1 ( Equation 1 ) ##EQU00001##

In this equation, z denotes a z conversion variable, T denotes a
coefficient given from pitch coefficient setting section 109 and suppose
M-1.

[0064]As shown in this figure, first spectrum S1(k) is stored in band
0≦k<FL of S(k) as the internal state of the filter. On the
other hand, estimated value S'2(k) of the second spectrum obtained from
the following procedure is stored in band FL≦k<FH of S(k).

[0065]A spectrum expressed by the following equation (2) is substituted in
S'2(k) thorough filtering processing. The substituted spectrum is
obtained by adding all spectrum βiS(k-T-i), obtained by
multiplying nearby spectrums S(k-T-i) separated by i centered on the
spectrum S(k-T) having a frequency lower than k by T by predetermined
weighting factor βi.

At this time, suppose the input signal given to this filter is zero. That
is, (Equation 2) expresses a zero input response of (Equation 1).
Estimated value S'2(k) of the second spectrum in FL≦k<FH is
calculated by performing the above described calculations while changing
k within a range FL≦k<FH in ascending order of frequencies
(from k=FL).

[0066]The above described filtering processing is performed within range
F≦k<FH every time pitch coefficient T is given from pitch
coefficient setting section 109 by clearing S(k) to zero every time. That
is, S(k) is calculated every time pitch coefficient T changes and output
to search section 108.

[0067]Next, calculation processing of the degree of similarity performed
by search section 108 and derivation processing of optimum pitch
coefficient T will be explained.

[0068]First, there are various definitions of the degree of similarity.

[0069]Here, a case where the degree of similarity defined by the following
equation based on a least square error method is used assuming that
filter coefficients β-1 and β1 are 0 will be
explained as an example.

In the case where this degree of similarity is used, filter coefficient
βi is determined after optimum pitch coefficient T is
calculated. Here, E denotes a square error between S2(k) and S'2(k). In
this equation, the first term of the right side becomes a fixed value
which is irrelevant to pitch coefficient T, and therefore pitch
coefficient T for generating S'2(k) which makes a maximum of the second
term of the right side is searched. The second term of the right side of
this equation will be called a "degree of similarity."

[0070]FIG. 7A to FIG. 7E are diagrams illustrating how the spectrum of
estimated value S'2(k) of the second spectrum changes as pitch
coefficient T changes.

[0071]FIG. 7A is a diagram illustrating the first spectrum having a
harmonic structure stored as an internal state. Furthermore, FIG. 7B to
FIG. 7D are diagrams illustrating spectra of estimated values S'2(k) of
the second spectrum calculated by performing filtering using three types
of pitch coefficients T0, T1, T2. FIG. 7E shows second
spectrum S2(k) to be compared with the spectrum of estimated value
S'2(k).

[0072]In the example shown in this figure, the spectrum shown in FIG. 7C
is similar to the spectrum shown in FIG. 7E, and therefore it is realized
that the degree of similarity calculated using T1 shows the highest
value. That is, T1 is an optimum value as pitch coefficient T
whereby the harmonic structure can be maintained.

[0073]FIG. 8A to FIG. 8E domain also figures similar to FIG. 7A to FIG.
7E, but here the phase of the first spectrum stored as the internal state
is different from that of FIG. 7A to FIG. 7E. However, in the example
shown in this figure, pitch coefficient T whereby the harmonic structure
is maintained is also T1.

[0074]Thus, changing pitch coefficient T and finding T of a maximum degree
of similarity is equivalent to finding out a pitch (or an integer
multiple thereof) of the harmonic structure of the spectrum on a
try-and-error basis. The coding apparatus of this embodiment calculates
estimated value S'2(k) of the second spectrum based on the pitch of this
harmonic structure, and therefore the harmonic structure does not
collapse in the connection area between the first spectrum and estimated
spectrum. This is easily understandable considering that estimated value
S'2(k) of the connection section when k=FL is calculated based on the
first spectrum separated by pitch (or an integer multiple thereof) T of
the harmonic structure.

[0075]Furthermore, pitch coefficient T expresses an integer multiple
(integer value) of the frequency interval of the spectrum data. However,
the pitch of the actual harmonic structure is often a non-integer value.
Therefore, by selecting appropriate weighting factor βi and
applying a weighted addition to M neighboring data centered on T, it is
possible to express a pitch of the harmonic structure of a non-integer
value within a range from T-M to T+M.

[0076]FIG. 9 is a flow chart showing an example of a series of algorithms
of processes performed by filtering section 107, search section 108 and
pitch coefficient setting section 109. An overview of these processes has
already been explained, and therefore detailed explanations of the flow
will be omitted.

[0077]Next, the calculation processing of a filter coefficient by filter
coefficient calculation section 110 will be explained.

[0079]Filter coefficient calculation section 110 holds a combination of a
plurality of βi(i=-1,0,1) as a data table beforehand,
determines a combination of βi(i=-1,0,1) that minimizes square
distortion E of above described (Equation 4) and outputs an index
thereof.

[0080]Thus, for the spectrum of an input signal divided into two parts of
a low-frequency domain (0≦k<FL) and high-frequency domain
(FL≦k<FH), the coding apparatus of this embodiment estimates
the shape of the high-frequency spectrum using filtering section 107 that
includes the low-frequency spectrum as the internal state, encodes and
outputs a parameter indicating the filter characteristic of filtering
section 107 instead of the high-frequency spectrum, and therefore, it is
possible to perform coding of the spectrum at a low bit rate and with
high quality.

[0081]Furthermore, in the above described configuration, when filtering
section 107 estimates the shape of the high-frequency spectrum using the
low-frequency spectrum, pitch coefficient setting section 109 changes the
frequency difference between the low-frequency spectrum which serves as a
reference for estimation and the high-frequency spectrum, that is, pitch
coefficient T, in various ways and outputs the frequency difference, and
search section 108 detects T corresponding to a maximum degree of
similarity between the low-frequency spectrum and high-frequency
spectrum. Therefore, it is possible to estimate the shape of the
high-frequency spectrum based on the pitch of the harmonic structure of
the overall spectrum and perform coding while maintaining the harmonic
structure of the overall spectrum.

[0082]Furthermore, there is no need for setting the bandwidth of the
low-frequency spectrum based on the pitch of the harmonic structure. That
is, it is not necessary to match the bandwidth of the low-frequency
spectrum to the pitch of the harmonic structure (or an integer multiple
thereof), and it is possible to set a bandwidth arbitrarily. This is
because the above described configuration allows spectra to be connected
smoothly in the connection section between the low-frequency spectrum and
high-frequency spectrum without matching the bandwidth of the
low-frequency spectrum to the pitch of the harmonic structure.

[0083]This embodiment has explained the case where M=1 in (Equation 1) as
an example, but M is not limited to this and an integer (natural number)
of 0 or greater can also be used.

[0084]Furthermore, this embodiment has explained the coding apparatus that
performs hierarchical coding (scalable coding) as an example, but above
described spectrum coding section 100 can also be mounted on a coding
apparatus that performs coding based on other schemes.

[0085]Furthermore, this embodiment has explained the case where spectrum
coding section 100 includes frequency domain conversion sections 104,
105. These are components necessary when a time domain signal is used as
an input signal, but the frequency domain conversion section is not
necessary in a structure in which the spectrum is directly input to
spectrum coding section 100.

[0086]Furthermore, this embodiment has explained the case where the
high-frequency spectrum is coded using the low-frequency spectrum, that
is, using the low-frequency spectrum as a reference for coding, but the
method of setting the spectrum which serves as a reference is not limited
to this, and it is also possible to perform coding of the low-frequency
spectrum using the high-frequency spectrum or perform coding of the
spectra of other regions using the spectrum of an intermediate frequency
band as a reference for coding though these are not desirable from the
standpoint of effectively using energy.

[0087]FIG. 10 is a block diagram showing the principal configuration of
radio reception apparatus 180 that receives a signal transmitted from
radio transmission apparatus 130.

[0091]FIG. 11 is a block diagram showing the internal configuration of
above described decoding apparatus 170. Here, a case where a signal
subjected to hierarchical coding is decoded will be explained as an
example.

[0093]RF demodulation apparatus 182 inputs digital demodulated coded
acoustic signal to input terminal 171. Separation section 172 separates
the demodulated coded acoustic signal input via input terminal 171 and
generates a code for first layer decoding section 173 and a code for
spectrum decoding section 150. First layer decoding section 173 decodes
the decoded signal having signal band 0≦k<FL using the code
obtained from separation section 172 and provides this decoded signal to
upsampling section 174. Furthermore, the other output is connected to
output terminal 176. This allows, when the first layer decoded signal
generated by first layer decoding section 173 needs to be output, the
first layer decoded signal can he output via this output terminal 176.

[0094]Upsampling section 174 increases the sampling frequency of the first
layer decoded signal provided from first layer decoding section 173.
Spectrum decoding section 150 is given the code separated by separation
section 172 and the upsampled first layer decoded signal generated by
upsampling section 174. Spectrum decoding section 150 performs spectrum
decoding which will be described later, generates a decoded signal having
signal band 0≦k<FH and outputs the decoded signal via output
terminal 177. Spectrum decoding section 150 regards the upsampled first
layer decoded signal provided from upsampling section 174 as the first
signal and performs processing.

[0095]According to this configuration, when the first layer decoded signal
generated by first layer decoding section 173 needs to be output, the
first layer decoded signal can be output from output terminal 176.

[0096]Furthermore, when an output signal of higher quality of spectrum
decoding section 150 needs to be output, the output signal can be output
from output terminal 177. Decoding apparatus 170 outputs either one of
signals output from terminal 176 or output terminal 177 and provides the
signal to D/A conversion apparatus 183. Which signal is to be output
depends on the setting of the application or judgment of the user.

[0099]A filter coefficient indicating a code obtained by spectrum coding
section 100 is input to input terminal 152 via separation section 172.
Furthermore, a first signal having an effective frequency band of
0≦k<FL is input to input terminal 153. This first signal is the
first layer decoded signal decoded by first layer decoding section 173
and upsampled by upsampling section 174.

[0101]Internal state setting section 155 sets the internal state of a
filter used in filtering section 156 using first spectrum S1(k).

[0102]Filtering section 156 performs filtering of the first spectrum based
on the internal state of the filter set by internal state setting section
155 and pitch coefficient T' and filter coefficient β provided from
input terminal 152 and calculates estimated value S'2(k) of the second
spectrum. In this case, filtering section 156 uses the filter function
described in (Equation 1).

[0103]Time domain conversion section 158 converts decoded spectrum S'(k)
obtained from filtering section 156 to a time domain signal and outputs
the decoded spectrum via output terminal 159. Here, processing such as
appropriate windowing and overlapped addition is performed as required to
avoid discontinuation that may occur between frames.

[0105]As shown in this figure, decoded spectrum S'(k) having frequency
band 0≦k<FL consists of first spectrum S1(k) and decoded
spectrum S'(k) having frequency band FL≦k<FH consists of
estimated value S'2(k) of the second spectrum.

[0106]Thus, the decoding apparatus of this embodiment has the
configuration corresponding to the coding method according to this
embodiment, and therefore, it is possible to decode a coded acoustic
signal efficiently with fewer bits and output an acoustic signal of high
quality.

[0107]Here, the case where the coding apparatus or decoding apparatus
according to this embodiment is applied to a radio communication system
has been explained as an example, but the coding apparatus or decoding
apparatus according to this embodiment is also applicable to a wired
communication system as shown below.

[0108]FIG. 14A is a block diagram showing the principal configuration of
the transmitting side when the coding apparatus according to this
embodiment is applied to a wired communication system. The same
components as those shown in FIG. 3 are assigned the same reference
numerals and explanations thereof will be omitted.

[0112]FIG. 14B is a block diagram showing the principal configuration of
the receiving side when the decoding apparatus according to this
embodiment is applied to a wired communication system. The same
components as those shown in F1G. 10 are assigned the same reference
numerals and explanations thereof will be omitted.

[0114]The input terminal of reception apparatus 191 is connected to
network N1. The input terminal of decoding apparatus 170 is connected to
the output terminal of reception apparatus 191. The input terminal of D/A
conversion apparatus 183 is connected to the output terminal of decoding
apparatus 170. The input terminal of output apparatus 184 is connected to
the output terminal of D/A conversion apparatus 183.

[0116]Thus, according to the above described configuration, it is possible
to provide a wired transmission/reception apparatus having operations and
effects similar to those of the above described radio
transmission/reception apparatus.

Embodiment 2

[0117]FIG. 15 is a block diagram showing the principal configuration of
spectrum coding section 200 in a coding apparatus according to Embodiment
2 of the present invention. This spectrum coding section 200 has a basic
configuration similar to that of spectrum coding section 100 shown in
FIG. 5 and the same components are assigned the same reference numerals
and explanations thereof will be omitted.

[0118]A feature of this embodiment is to make a filter function used in
the filtering section simpler than that in Embodiment 1.

[0119]For the filter function used in filtering section 201, a simplified
one as shown in the following equation is used.

[0121]FIG. 16 illustrates an overview of filtering using the above
described filter.

[0122]Estimated value S'2(k) of a second spectrum is obtained by
sequentially copying low-frequency spectra separated by T. Furthermore,
search section 108 determines optimum pitch coefficient T' by searching
for pitch coefficient T which minimizes E of (Equation 3) as in the case
of Embodiment 1. Pitch coefficient T' obtained in this way is output via
output terminal 111. In this configuration, the characteristic of the
filter is determined only by pitch coefficient T.

[0123]Note that the filter of this embodiment is characterized in that it
operates in a way similar to an adaptive codebook, one of components of a
CELP (Code-Excited Linear Prediction) scheme which is a representative
technology of low-rate voice coding.

[0124]Next, the spectrum decoding section that decodes a signal coded by
above described spectrum coding section 200 will be explained (not
shown).

[0125]This spectrum decoding section has a configuration similar to that
of spectrum decoding section 150 shown in FIG. 12, and therefore detailed
explanations thereof will be omitted, and it has the following features.
That is, when filtering section 156 calculates estimated value S'2(k) of
the second spectrum, it uses the filter function described in (Equation
5) instead of the filter function described in (Equation 1). It is only
pitch coefficient T' that is provided from input terminal 152. That is,
which of the filter function described in (Equation 1) or (Equation 5)
should be used is determined depending on the type of the filter function
used on the coding side and the same filter function used on the coding
side is used.

[0126]Thus, according to this embodiment, the filter function used in the
filtering section is made simpler, which result in eliminating the
necessity for installing a filter coefficient calculation section.
Therefore, it is possible to estimate the second spectrum (high-frequency
spectrum) with a smaller amount of calculation and also reduce the
circuit scale.

Embodiment 3

[0127]FIG. 17 is a block diagram showing the principal configuration of
spectrum coding section 300 in a coding apparatus according to Embodiment
3 of the present invention. This spectrum coding section 300 has a basic
configuration similar to that of spectrum coding section 100 shown in
FIG. 5 and the same components are assigned the same reference numerals
and explanations thereof will be omitted.

[0128]A feature of this embodiment is to further comprise outline
calculation section 301 and multiplexing section 302 and perform coding
of envelope information about a second spectrum after estimating the
second spectrum.

[0129]Search section 108 outputs optimum pitch coefficient T' to
multiplexing section 302 and outputs estimated value S'2(k) of the second
spectrum generated using this pitch coefficient T' to outline calculation
section 301. Outline calculation section 301 calculates envelope
information about second spectrum S2(k) based on second spectrum S2(k)
provided from frequency domain conversion section 105. Here, a case where
this envelope information is expressed by spectrum power for each subband
and frequency band FL≦k<FH is divided into J subbands will be
explained as an example. At this time, the spectrum power of the jth
subband is expressed by the following equation.

In this equation, BL(j) denotes a minimum frequency of the jth
subband, BH(j) denotes a maximum frequency of the jth subband. The
subband information of the second spectrum obtained in this way is
regarded as the spectrum envelope information about the second spectrum.

[0130]In a similar fashion, subband information B'(j) of estimated value
S'2(k) on the second spectrum is calculated according to the following
equation,

[0132]Thus, this embodiment makes it possible to improve an accuracy of
the estimated value of the high-frequency spectrum since the envelope
information about the high-frequency spectrum is further coded after a
high-frequency spectrum is estimated.

Embodiment 4

[0133]FIG. 18 is a block diagram showing the principal configuration of
spectrum decoding section 550 according to Embodiment 4 of the present
invention. This spectrum decoding section 550 has a basic configuration
similar to that of spectrum decoding section 150 shown in FIG. 12, and
therefore the same components are assigned the same reference numerals
and explanations thereof will be omitted.

[0134]A feature of this embodiment is to further comprise separation
section 551, spectrum envelope decoding section 552 and spectrum
adjusting section 553. This allows spectrum coding section 300 or the
like shown in Embodiment 3 to perform decoding of a code resulting from
coding of envelope information as well as coding of an estimated spectrum
of a high-frequency spectrum.

adjusts a spectral shape in frequency band FL≦k<FH of decoded
spectrum S'(k) and generates adjusted decoded spectrum S3(k). This
adjusted decoded spectrum S3(k) is output to time domain conversion
section 158 and converted to a time domain signal.

[0138]Thus, according to this embodiment, it is possible to decode a code
including envelope information.

[0139]This embodiment has explained the case where the spectrum envelope
information provided from separation section 551 is value Vq(j)
obtained by coding amount of variation V(j) for each subband shown in
(Equation 8) as an example, but the spectrum envelope information is not
limited to this.

Embodiment 5

[0140]FIG. 19 is a block diagram showing the principal configuration of a
spectrum decoding section 650 in a decoding apparatus according to
Embodiment 5 of the present invention. This spectrum decoding section 650
has a basic configuration similar to that of spectrum decoding section
550 shown in FIG. 18, and therefore the same components are assigned the
same reference numerals and explanations thereof will be omitted.

[0141]A feature of this embodiment is to further comprise LPC spectrum
calculation section 652, use an LPC spectrum calculated with an LPC
coefficient as spectrum envelope information, estimate a second spectrum,
and then multiply the second spectrum by the LPC spectrum to obtain a
more accurate estimated value of the second spectrum.

Here, NP denotes the order of the LPC coefficient. Furthermore, it is also
possible to calculate LPC spectrum env(k) using variable
γ(0<γ<1) and changing the characteristic of the LPC
spectrum. In this case, LPC spectrum env(k) is expressed by the following
equation.

adjusts the spectrum in frequency band FL≦k<FH of decoded
spectrum S'(k) and generates adjusted decoded spectrum S3(k). This
adjusted decoded spectrum S3(k) is provided to time domain conversion
section 158 and converted to a time domain signal.

[0144]Thus, according to this embodiment, using an LPC spectrum as
spectrum envelope information makes it possible to obtain a more accurate
estimated value of the second spectrum.

[0145]The coding apparatus or decoding apparatus according to the present
invention can be mounted on a communication terminal apparatus and base
station apparatus in a mobile communication system, and therefore, it is
possible to provide a communication terminal apparatus and base station
apparatus having operations and effects similar to those described above.

[0146]The case where the present invention is constructed by hardware has
been explained as an example so far, but the present invention can also
be implemented by software.

[0147]The present application is based on Japanese Patent Application No.
2003-323658 filed on Sep. 16, 2003, entire content of which is expressly
incorporated by reference herein.

INDUSTRIAL APPLICABILITY

[0148]The coding apparatus and decoding apparatus according to the present
invention have the effect of performing coding at a low bit rate and is
also applicable to a radio communication system or the like.